How do structures carry loads, and how do forces, moments and reinforcement keep a product strong and stable?
Structures and forces: types of structure (frame, shell, monocoque), the forces of tension, compression, shear, bending and torsion, the principle of moments and equilibrium, and methods of reinforcing and stiffening to improve strength and rigidity.
A focused answer to OCR A-Level Product Design on structures and forces: frame, shell and monocoque structures, the forces of tension, compression, shear, bending and torsion, the principle of moments and equilibrium with worked calculations, and methods of reinforcing and stiffening.
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What this dot point is asking
OCR wants you to know the types of structure, the forces that act on them, the principle of moments and equilibrium (with calculation), and how to reinforce and stiffen a structure. This is the structural half of the technical principles, and it carries calculation marks.
Types of structure
The forces acting on structures
The principle of moments and equilibrium
The exam point is to take moments about a sensible pivot (one that removes an unknown), then set clockwise equal to anticlockwise.
Reinforcing and stiffening
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksA uniform shelf bracket is pivoted at the wall. A 60 N load hangs 0.30 m from the pivot, and a support strut provides an upward force at 0.10 m from the pivot. Calculate the force in the strut needed to keep the bracket in equilibrium.Show worked answer →
A Component 01 moments calculation. Marks for the method (moments balance), the substitution and the answer with units.
Use the principle of moments: for equilibrium, the sum of clockwise moments equals the sum of anticlockwise moments about the pivot. The load gives a clockwise moment N m. The strut gives an anticlockwise moment . Setting them equal: , so N.
A common dropped mark is omitting the unit (N) or using the wrong distances; the moment is force times the perpendicular distance from the pivot, and the two moments must balance for equilibrium.
OCR 20216 marksExplain, with examples, three ways a designer can reinforce or stiffen a structure without greatly increasing its mass.Show worked answer →
A Component 01 question marked by points within a levels structure. One developed method per mark pair.
Award marks for any three, developed: triangulation, adding diagonal members to turn a deformable rectangular frame into rigid triangles, which resist racking (used in shelf brackets, bridges and chair frames). Using a tube or hollow section instead of solid, because a tube has a high stiffness-to-weight ratio (most material is far from the neutral axis, resisting bending) so a bicycle frame is stiff yet light. Folding, ribbing or corrugating a sheet, because bends, ribs and corrugations greatly increase the second moment of area and so the bending stiffness of thin sheet (corrugated card, ribbed plastic mouldings, folded metal panels). Lamination and adding webs or flanges (an I-section) also score.
A common dropped mark is naming a method without explaining why it adds stiffness for little extra mass.
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